main.dart

  void main() => runApp(DemoApp());

  class DemoApp extends StatelessWidget {
    const DemoApp();

    @override
    Widget build(BuildContext context) {
      return ChangeNotifierProvider<DownloadProgress>(
        create: (_) => DownloadProgress(),
        child: MaterialApp(
          home: Scaffold(
            appBar: AppBar(
              title: const Text("Download Demo"),
            ),
            body: Center(
              child: const DownloadWidget(),
            )
          )
        ),
      );
    }
  }
  

progress.dart

  class DownloadProgress with ChangeNotifier {

    double get progress => _progress;

    void start({ required String url,  required String localPath}) async {
      // Reset in case it isn't at zero
      _resetProgress();

      // Path and name
      final directory = await getTemporaryDirectory();
      final fileName = "${directory.path}/$localPath";

      // Donwload the file
      await Dio().download(url, fileName,
        options: Options(
          headers: {
            HttpHeaders.acceptEncodingHeader: "*"
          }
        ),
        onReceiveProgress: (received, total) {
          if (total != -1) {
            var pos = received / total * 100;
            _updateProgress(pos);
          }
        }
      );

    }

    double _progress = 0;

    void _resetProgress() {
      if (progress != 0) {
        _progress = 0;
        notifyListeners();
      }
    }

    void _updateProgress(double value) {
      _progress = value;
      notifyListeners();
    }

  }
  

download_widget.dart

  class DownloadWidget extends StatelessWidget {
    final String url = "https://website.com/files/something.pdf";
    const DownloadWidget();

    @override
    Widget build(BuildContext context) {
      return Center(
        child: Consumer<DownloadProgress>(
          builder: (context, status, _) {
            var progress = status.progress.toStringAsFixed(1);

            return RaisedButton(
              child: Text("$progress %"),
              onPressed: () => status.start(url: url,
                localPath: "file_p2.pdf"
              ),
            );
          },
        ),
      );
    }

  }